The present invention relates to the arc suppression art. It finds particular application in conjunction with suppressing arcing of x-ray tubes in CT scanners operating at high voltages, and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in connection with x-ray tubes for other applications, and the like.
An x-ray tube commonly has a source filament which is heated to boil-off electrons and a target which emits x-rays when bombarded by a beam of the electrons. An electronic power supply provides a high voltage across the electron source and the target in order to draw the electrons towards the target in a beam with sufficient energy to cause the generation of x-rays. Conventionally, the power supply includes electronic controls for selecting the potential applied between the source and the target, known as the kV, and the milliamperes of current flowing between the target and the anode, commonly known as the mA. Typically, the x-ray tube of a high performance CT scanner has about 150 kV applied across the source and the target and controls the tube current between about 15-400 mA.
As might be expected, when two elements with 150 kV difference in potential are placed proximate to each other, there is a tendency to arc. In x-ray tubes, this tendency to arc often increases as the tube ages due to such factors as a degradation of the vacuum within the tube. When the x-ray tube arcs, a current on the order of hundreds of amperes can flow between the source and the anode. Once an x-ray tube starts to arc, an avalanche type effect occurs sputtering metal and the metal atoms as well as ionizing the contaminants in the vacuum, reducing resistance, and allowing the current to build very rapidly, on the order of microseconds, to hundreds of amperes where generators of high output capacitance are used. The arcing problem can be exaggerated with high frequency multiplier type generators, as are commonly used on high performance high voltage x-ray tubes since these generators, by necessity, are high capacitance type.
Typically, the electron source or cathode filament is connected with a -75 kV output of the generator and the anode target is connected with a +75 kV output of the generator. In this manner, a 150 kV potential is maintained between the source and the anode, yet each is at a potential of only 75 kV relative to ground.
In order to limit arcing, others have placed a resistor between the negative output of the generator and the electron source and another resistor between the positive output of the generator and the anode target. If the resistor is too large, there is a significant drop in the potential between the electron source and the anode target. If the resistor is too small, it has a negligible effect on arcing. An acceptable value for the resistance is about 350 Ohms. A 350 Ohm resistor in a 150 kV generator limits the arc current to about 210 amps (75,000 V/350 .OMEGA.). A current of this magnitude is not only detrimental to the x-ray tube, but produces a distinct discontinuity in the production of x-rays. In a CT scan, several views of bad or missing data result.
The inventors herein have determined that by limiting the arcing current at the beginning of an arc, the avalanche type effect can be avoided, and severe arcing can be minimized.